In the electrophotographic process, especially the xerographic process, and in magnetic imaging systems similar steps are involved in causing the formation and development of images, including for example the formation of a latent image, the development of the latent image with electromagnetic materials, such as toner, optionally, transferring the developed image to a suitable support such as paper, fusing the image to the paper substrate using a number of known techniques, including those employing heat, and optionally cleaning the surface from which the developed latent image has been transferred. In the xerographic process the photoconductive surface which contains an electrostatic latent image can be developed by means of a variety of pigmented resin materials specifically made for this purpose, such as toners. The toner material is electrostatically attracted to the latent image on the plate in proportion to the charge concentration thereon. These toner materials can be applied by a number of known techniques including for example, cascade development, reference U.S. Pat. No. 3,618,552, magnetic brush development, reference U.S. Pat. No. 2,874,063, and touchdown development, reference U.S. Pat. No. 3,166,432. The developed image is then transferred to a suitable substrate such as paper and can be fixed by using a number of different techniques including for example vapor fixing, heat fixing, pressure fixing or combinations thereof as described for example in U.S. Pat. No. 3,539,161.
In magnetic imaging systems substantially the same process steps are involved as described above with respect to electrophotographic imaging systems, thus there is formed a latent magnetic image on a magnetizable recording medium, which image can be used in duplicating processes, for example, by repetitive toning, and transfer of the developed image. The latent magnetic image is formed by any suitable magnetization procedure whereby a magnetizable layer of marking material is magnetized and such magnetism transferred imagewise to the magnetic substrate. The latent magnetic image can be developed with a magnetic developer to render such image visible. The developed visible magnetic image can then be typically tranferred to a receiver such as for example paper, which image is fused on the paper, in order to produce a final copy or print referred to in the art as a hard copy. There are a number of known techniques for creating the latent imate which are described for example in U.S. Pat. Nos. 4,032,923; 4,060,811; 4,074,276; 4,030,105; 4,035,810; 4,101,904; and 4,121,261, the teachings of these patents being completely incorporated herein by reference.
One method of developing magnetic images is referred to as magnetic toner touchdown development, which involves providing a substantially uniform layer of toner comprising magnetic material on a conductive substrate, which material can be brought either closely adjacent to that of the image or in contact with the image. The magnetic material in the toner acts as an extension of the conductive backing and therefore acquires charge, induced therein by the latent image of a polarity opposite to that in the latent image. The conductive substrate can be biased to assist in transfer of the toner to the latent image, however, a conductive backing is not essential.
Typical suitable fusing methods that may be used have been described in the prior art and include for example, heating the developed image (toner) to cause the resins thereof to at least partially melt and become adhered to the photoconductor binder member or copy substrate in the case of images transferred from the imaging media, followed by the application of pressure to the toner with heating such as the use of a heated roller. Solvent or solvent vapor fusing has also been used, wherein the resin component of the toner is partially dissolved. The photoconductor binder member or copy substrate is typically of sufficient hardness to allow fixing solely by the application of pressure such as for example by a contact roller and in an amount sufficient to calender the toner. With some existing toner materials, images are fixed using a heat pressure fusing system at surface speeds of up to 20 inches per second but recently it has been found desirable to achieve higher fixing speeds and special toner materials are needed in order to effect such high fixing speeds particularly in magnetic systems where the high magnetic pigment loading required for development can have an adverse effect on the desired fusing and fixing levels of the toner.
Concurrently with the growth of interest in magnetic imaging there has been increased interest in magnetic developers to render the latent magnetic images visible. In U.S. Pat. No. 3,221,315 there is described the use of encapsulated ferrofluids in a magnetic recording medium, wherein the ferrofluid orientation in the presence of a magnetic field exhibits a variable light responsive characteristic. In this situation the magnetic recording medium is self-developing in the sense that magnetic marking material need not be employed to render a visible image. In other situations latent magnetic images are rendered visible by magnetic marking materials. Thus, for example, in U.S. Pat. No. 3,627,682 there is disclosed binary toners for developing latent magnetic images, which binary toners include a particulate hard magnetic material and a particulate soft magnetic material in each toner particle. The toner particles include two materials in a binder material. In U.S. Pat. No. 2,826,634 there is described the use of iron or iron oxide particles either alone or encapsulated in low melting resin or binders for developing latent magnetic images. Low optical density and relative unresponsiveness to weak magnetic fields are exhibited by relatively large iron or iron oxide base magnetic particles.
Other patents evidencing the continuing interest in improved magnetic developers include U.S. Pat. No. 3,520,811, which discloses that magnetic particles of chromium dioxide appear to catalyze a surface polymerization or organic air drying film forming vehicles such as those employed in oil base materials in order that a coating of polymerized vehicle is formed around the particle; and U.S. Pat. No. 3,905,841 which teaches the prevention of agglomeration and the formation of homogeneous dispersions of cobalt-phosphorous particles into an organic resin binder by treatment with a solution containing sulfuric acid.